This invention is concerned with the electrolytic refining of copper, and more particularly, with a process for constantly maintaining an effective thiourea concentration in the electrolyte solution during the electrolytic refining of copper.
Traditionally, copper has been purified in a process wherein an electric current is transmitted between cast anodes of impure copper to cathodes which have a substantially pure copper deposit plated thereto, both anodes and cathodes being immersed in a suitable electrolyte. The electrolyte which has found universal acceptance in the art is an aqueous solution of copper sulfate and sulfuric acid. The refining process first dissolves the impure anode copper into the electrolyte solution and then transports the copper ions (Cu.sup.2+) to a nearby cathode, where the copper is plated out as virtually pure metal, Cu.degree.. After a period of time a desired thickness of copper is deposited on the cathodes, whereupon they are removed and later melted for casting into several common product shapes.
Several problems occur during this operation and have consequently formed the subject of extensive research. As the cost of energy continues to soar, the importance of increasing current efficiency in electrorefining has become a paramount consideration. A change of 1% in the current efficiency of a large modern electrolytic copper refinery can result in a substantial increase in copper capacity, or a decrease in electricity per unit of production. Additionally, it is desired to operate at higher tankhouse current densities without sacrificing current efficiency. Such an improvement would permit a larger and quicker recovery of copper, as well as several desirable by-products, e.g., silver, and would also reduce the need for shift work, thus lowering the cost of labor.
Various addition agents, such as those disclosed in U.S. Pat. Nos. 2,660,555 and 3,389,064, have assisted in improving the quality of the copper deposited on the cathodes. In particular, the addition of a combination of glue, Avitone and thiourea; "thiourea" hereafter being understood to represent either pure or commercial grade thiourea; as well as most organic compounds which contain a thiourea group, as disclosed in U.S. Pat. No. 3,389,064 has been found to favor the formation of a smooth, dense, uniform cathode copper deposit. Without the use of such additives the copper deposited on the cathodes tends to develop "nodules", which are irregular, tree-like growths that often cause harmful short circuits in the process. Also, large "striations", which are groovelike growths in the cathode can trap impurities present in the electrolyte, and are particularly dangerous when the impurity concentration, and it has been thought, especially the thiourea concentration, rise to undesirable levels in the electrolyte.
A problem created by the use of additives has been the need to quickly and accurately determine optimum operating concentrations in the refining tankhouse, and also how to maximize the current efficiency during the plating process. U.S. Pat. No. 3,389,064 does not reveal the chemistry of the disclosed additives in the electrolyte, but rather assumes that the addition agents appear to be used up during the electrolytic refining process. However, in any large commercial process, including tankhouse refining, successful operation can depend on a number of variables, and thus it is desired to discover a method of quickly and accurately measuring the critical parameters in the system in order to keep it constantly running under maximum operating conditions, while not having to resort to old fashioned trial-and-error readjustments, and also determining the ranges of additive concentrations in which copper refining should be conducted.